1. Field of the Invention
The present invention relates to a voltage detection circuit which is designed such that a voltage input into a voltage detection IC (Integrated Circuit) does not fall below a minimum operating voltage, so as to be capable of detecting the voltage correctly, while preventing malfunction of the voltage detection IC.
2. Background Art
A voltage detection circuit as shown in FIG. 4 is known as an example of conventional voltage detection circuits.
The voltage detection circuit shown in FIG. 4 will be described hereinafter. The voltage detection circuit 2 has a terminal A and a terminal B, and an external constant voltage source is connected to the terminal A, and a power source for measuring a voltage is connected to the terminal B.
The voltages Va and Vb are applied to terminals A and B, respectively, and the voltage Vb is input into the voltage detection circuit. The output of the voltage detection IC 1 is connected to the terminal A through the pull-up resistor R3. The result of the voltage detection IC 1 is output at a terminal C.
Here, an operation of the voltage detection circuit will be described. As described, the external constant voltage source is connected to the terminal A, and the voltage Va is applied to the terminal A. The terminal B is connected to the power source for measuring the voltage and the voltage Vb is applied to the terminal B.
The terminal B is connected to the voltage detection IC 1, and the voltage Vb is input into the voltage detection IC 1. The output end of the voltage detection IC 1 and the resistor R3 are connected to the terminal C, and the voltage Va is applied to the pull-up resistor R3.
When the voltage Vb is higher than a detection voltage, a voltage at a level of "H" is output at the terminal C, and when the voltage Vb is lower than the detection voltage, a voltage at a level of "L" is output at the terminal C.
Next, the operation is described in more detail using a wave-form diagram shown in FIG. 5. FIG. 5 is a diagram to explain the change of the input and output voltages of the voltage detection circuit.
FIG. 5A shows an input voltage Vb to the terminal B, and the input voltage Vb is input into the voltage detection IC 1. FIG. 5B shows an output voltage of the voltage detection circuit which appears at the terminal C.
In the above figures, it is assumed that the detection voltage of the voltage detection IC 1 is 2.5 V, the minimum operating voltage of the voltage detection IC 1 is 0.5 V, and the voltage Va at the terminal A is 3.5 V.
The region a is a range wherein the voltage Vb is higher than the detection voltage of the voltage detection IC 1. Since the voltage Vb to be input into the voltage detection IC 1 is higher than the detection voltage of the voltage detection IC 1, the level "H" is output at the terminal A.
The region b is a range wherein the voltage Vb is lower than the detection voltage of the voltage detection IC 1, but higher than the minimum operating voltage. Since the voltage Vb to be input in the voltage detection IC 1 is lower than the detection voltage of the voltage detection IC 1, the level "L" is output at the terminal C.
The region c is a range wherein the voltage Vb is lower than the minimum operating voltage of the voltage detection IC 1. Despite the voltage Vb to be input in the voltage detection IC 1 being lower than the detection voltage of the voltage detection IC 1, since the voltage Vb is lower than the minimum operating voltage of the voltage detection IC 1, the terminal voltage becomes unstable and causes a malfunction of the voltage detection IC 1.
The region d is, similar to the region b, a range wherein the voltage Vb is lower than the detection voltage of the voltage detection IC 1. Since the voltage Vb to be input into the voltage detection IC 1 is lower than the detection voltage of the voltage detection circuit IC 1, the level "L" is output at the terminal C.
The region e is, similar to the region a, a range wherein the voltage Vb is higher than the detection voltage of the voltage detection IC 1. Since the voltage to be input in the voltage detection IC 1 is higher than the detection voltage of the voltage detection IC 1, the level "H" is output at the terminal C. Due to the effect of the pull-up resistor R3, the voltage which appears at the terminal C is Va.
As a result, change of the output voltage of the voltage detection IC 1 is obtained as shown in FIG. 5B. The region wherein the output of the voltage detection IC 1 changes is between regions a and b as well as between regions d and e.
However, in the region c, despite the fact that the voltage Vb is lower than the minimum operating voltage of the voltage detecting IC 1, and that the voltage Vb is lower than the detection voltage, a problem arises that it is not possible to execute a normal detection of the voltage in the region c and causes malfunction of the voltage detection IC 1.
Although a reference voltage generating circuit which is capable of setting a very accurate detection level of a source voltage detection circuit is disclosed in Japanese Patent Application, First Publication No. Hei 06-258359, the above described problem is still remains unsolved.